# Design49 — flat landing pad at tunnel ends (owner idea, reviewed 2026-11-02)

Owner: "the end of the tunnel should be flat; flatten the arrival with a fade-out that blends to the surrounding
density; a cheap stamp applied at arrival, no matter what's there."

A read-only design review (Claude Opus 5.5, medium effort) of `wip/l134-cave-planner` recommends a **clamp, not a
lerp**. Each tunnel end gets a level disc at the tunnel's end floor P. Out to a blend radius, the native ground is
squeezed between two gentle cones (12°) starting at P: a lower one that fills solid, an upper one that carves air.
Inside the disc the floor is exactly flat; outside it, the slope is never steeper than max(cone, native), with no
overhang or cliff. The full note is in the session transcript; key points:

- **Placement:** the last step of `ApplyResolvedStrateComposition`, after the passage composer, on both the native and
  non-native branches. The fill is masked by the tube-only distance (bore stays open). It replaces the landing-chamber
  box, shell and floor-shell special cases that caused L152–L156.
- **Formula** (metres; ×cos α for 1-Lipschitz). d = max(0, r−R), t = tan α.
  - Fill: `Lo = P − t·d`, `F = max(z−Lo, (Lo−Ds)−z, r−(R+B))`.
  - Air: `Hi = P + t·d`, `A = min(z−Hi, (Hi+H)−z, (R+B)−r)`.
  - Result: `c = max(min(native, max(F, −tubeDist)), A)`.
  - Hard min/max inside R; smooth-min width ramps 0 → 0.5 m over the first metre outside it.
  - Why not lerp: native density saturates at about ±4.125 voxels (`DetailSupremum`), so a lerp makes cliffs and
    films.
- **Parameters:** α = 12°, R = max(mouthRadius + 1 m, 3 m), B = min(8 m, |Δh_ring|/t + 1 m), H = 3 m, Ds = 1.25 m.
- **Planner:**
  - P = h(end), with h′ = 0 over the last R.
  - Sample the edge ring; move P into the feasible band or take the next deterministic site.
  - Rewrite route heights inside R+B to the clamped ground.
  - Content-band check including P−tB−Ds and P+tB+H.
- **Bounds:** the pad influence box goes into the proof bounds, the fallback overlap test, the **tile feature
  bounds** (critical: the composer early-returns on an empty route list) and the proof-miss sampler.
- **Audits:** new `LandingPadAudit` (flatness ±2 cm, support, headroom, 36 radial blend transects ≤ 15° with one
  crossing) plus negative controls (pad off, 2% tilt, B halved, bore mask removed).
- **Risks:** caves under pads (connectivity), low ceilings, shaft footprints, island edges and their air gap, liminal
  decks; off-route edges on non-heightfield layers are a metric only.